The benefits of physical activity to the health and well-being of the general population and the risks of inactivity are now well documented. However, evidence concerning physical activity levels has focused largely on adults. Relatively little is known about the physical activity patterns of adolescents. The purpose of this study was to analyze levels of regular physical activity in representative samples of adolescents selected from a secondary school in the city of Londrina, Paraná, in the southern region of Brazil. A total of 281 adolescents (157 girls and 124 boys) ages ranging from 15 to 18 years were included in the study. The levels of regular physical activity were determined by means of a self-report instrument which provided an estimate of energy expenditure and of the time spent in different activities. Results showed that boys were consistently more active than girls. A percentage of 54% of the boys was considered active or moderately active based on their estimated daily average energy expenditure (kcal/kg/day), while approximately 65% of the girls were found to be inactive or very inactive. Boys spent a significantly longer time in physical exercise and in sports than girls (3:20 vs 0:48 hours/week). Levels of regular physical activity decreased significantly with age, and the decline was greater in girls. Girls spent less time watching TV than boys (3.3 vs 4.0 hours/days). Boys, but not girls, from lower socioeconomic families performed less physical activity than boys from higher socioeconomic families. Most adolescents (girls 97%; boys 74%) did not practice sufficient physical exercise so as to achieve the health benefits suggested by current guidelines and recommendations for young people. In conclusion, the levels of physical inactivity reported constitute an important issue for social agencies and organizations involved in the promotion of health-enhancing physical activity.